**6. Water absorption**

Water absorption is the most important aspect considering the usage of the fiber polymer composite material in various applications with different environmental conditions. Natural fibers are hydrophilic which lead to their mechanical failure during an application. For example, for a sandwich fiber polymer composites delamination between fiber part and a polymer could ensue as a result of moisture absorption. This is directly dependent on the polymer-type, temperature, humidity, fiber loading, orientation, fiber-matrix adhesion, and permeability of the fibers [35, 50, 51]. On the other hand, surface modification of the fibers may improve the interfacial adhesion between the fibers and the polymer matrix which in turn enhance water absorption resistance. This apparently emanated from the hydrophobicity of the fillers and interaction with the hydroxyl groups on the surface of the fillers, thus decreasing the overall water absorption of the composites. Vilay et al. [49] reported that the treatment of the fibers with acrylic acid (AA) improves the water absorption resistance of the composites. The chemical treatment reportedly reduced the hydroxyl groups which improved adhesion between the fibers and polymeric matrix. The difference between the pith and rind on the water absorption was studied by Lee and Mariatti [50]. The bigger size of lumens in the pith-fibers facilitated the water absorption into the composite material when compared to the rind-based composites.
